The invention relates to an ion-conductive membrane for electrochemical applications on the basis of an aromatic polyimide- and co-polyimide polymer and its use.
Fuel cells are presently the subject of intense development work since they are promising alternatives for energy conversion. For mobile applications, the so-called polyelectrolyte membrane fuel cell (PEFC) has been found to be particularly suitable, see F. R. Kalhammer, P. R. Prokopius, V. P. Roan, G. E. Voecks, Status and Prospects of Fuel Cells as Automobile Engines, State of California Air Research Board, 1998. An essential criterion for bringing this technology to the market is the availability of suitable membranes with high proton conductivity, a low fuel, or respectively, energy carrier permeability (hydrogen or methanol) and a high chemical stability. Such membranes however must also be relatively inexpensive.
For fuel cells, so far, so called Nafion® membranes which are fluorinated membranes from DuPont or similar, membranes from Dow and Asahi have been commercially available and have been widely used. (O. Savadogo, J. New Materials for Electrochemical Systems 1 (1998) 47).
An essential disadvantage of these Nafion® membranes however is their price. Therefore various non-fluorinated membranes have been tested in the last years. Most of them are based on sulfonated polymers and copolymers. Membranes of sulfonated polysulfon, sulfonated polyether ketone, sulfonated polyphosphene and sulfonated polyamides are described in various publications, see Q Guo, P. N. Pintauro, H. Tang, S. O'Connor, Sulfonated and cross-linked polyphosphazene-based proton exchange membranes, J. Membrane Sci. 154 (1999) 175, E. Vallejo, G. Pourcelli, C. Gavach, R. Mercier and M. Pineri, Sulfonated polyimides as proton conductor exchange membranes. Physicochemical properties and separation H+/Mz+ by electrodialysis comparison with a perfluorosulfonic membrane, J. Membrane Sci. 160 (1999) 127; S. Faure, M. Pineri, P. Aldebert, R. Mercier, B. Sillion, U.S. Pat. No. 6,245,881 B1.
A further disadvantage of the Nafion® membranes resides in the loss of proton conductivity at temperatures above 100° C., because of the removal of water. However, operation at 100-150° C. would be advantageous in order to reduce the poisoning of the catalyst by CO. A polymer which is believed to be usable in this temperature range is polybenzimidazole, which is usually doped with phosphoric acid (R. F. Savinell, M. H. Litt, Protein conducting polymers prepared by direct acid casting, U.S. Pat. No. 5,716,727).
Polybenzimidazole (PBI) has also been modified by sulfonizing in order to increase the conductivity below 100° C. (D. J. Jones and J. Roziere, Recent advances in the functionalisation of polybenzimidazole and polyether ketone for fuel applications, J. Membrane Sci. 185 (2001) 41). The basic character of the imidazole groups plays in this case an essential role in the transport of protons in PBI membranes and in their good performance above 100° C.
Also sulfonated polyimides have already been examined for use in fuel cells (C. Genies, R. Mercier, B. Sillion, N. Cornet, G. Gebel, M. Pineri; Soluble sulfonated napthalenic polyimides as materials for proton exchange membranes, Polymer 42 (2001) 359-373; C. Genies, R. Mercier, B. Sillion, R. Petiaud, N. Cornet, G. Gebel, M. Pineri. Stability study of sulfonated phtalic and naphtalenic polyimide structures in aqueous medium. Polymer 42(2001) 5097-5105). The possibilities for synthesis are very flexible and a multitude of structures can be obtained. However, the membranes examined so far are inadequate in many respects.
It is the object of the present invention to provide an improved membrane, which can be used for electrochemical applications, particularly in connection with fuel cells.